There is no definitive answer to this scenario. However, there are serious considerations.

Your existing patient is minimally differentiated, multiply injured and clinically unstable. She requires urgent assessment and ongoing care in a major trauma hospital. You have sedated and muscle relaxed this patient and she is now entirely dependent on mechanical ventilation and advanced clinical decisions to ensure, for example, optimal cerebral perfusion.

The clinical condition and requirements of the second patient are, at this moment, unknown. He may be uninjured, intoxicated, have an occult medical presentation or have sustained unsurvivable injuries. Only a reasonably thorough assessment will provide some degree of differentiation. It is perhaps reasonable to briefly (less than 5 minutes) leave your patient in order to quickly assess the new patient but to let the land ambulance continue without the PHR team is a high-risk decision. What happens if the tracheal tube is dislodged and cannot be resited?

This scenario is not the same as a multi-casualty scene. You have entered into a doctor–patient relationship with a single patient and you are obliged to do your best for her. You have not yet entered into a relationship with the new patient. If his accident had occurred on the adjacent street, you would not even know about him. In addition, there will now already be other land ambulances on their way.